It is known to form a cross-sectional profile of a tubular member by a hydroforming process in which a fluid filled tubular member is placed within a cavity of a die and then the die is closed so that the tubular member is pinched within the die. Fluid pressure is then increased inside the tubular member to expand the tubular member outwardly against the cavity of the die to provide a tubular product having a die formed cross-sectional profile.
During tube hydroforming, large size punches of various shapes are used to create desired openings in the tubular member. These large size punches require similarly large activating hydraulic cylinders in order to overcome the forces needed for piercing the tubular member. Large bore hydraulic cylinders need a high volume of oil flow to operate, which is undesired.
As the number of hydroformed tube applications grows, the need for complex piercing also increases. Tubes are not only designed with more openings, but also with larger openings, which, in turn, need larger hydraulic cylinders for piercing. The need to reduce oil flow for hydraulic cylinder operation as well as to control internal form pressure is becoming more important. The hydroforming industry is constantly searching for ways to reduce the necessary force to pierce openings.
A first method of piercing used in hydroforming applications is hydropiercing. Upon completion of hydroforming, the tubular member is in intimate contact with the wall of the die. The hydroforming fluid is at a forming pressure, approximately 10,000 psi. A punch is attached to a hydraulically actuated cylinder. When the hydraulic cylinder is extended, the high-pressure fluid will support the tubular member around the circumference of the punch, allowing the metal to shear. The punch will shear a slug, which will fall as a loose piece and lay inside the formed tubular member. After the punch extends through the wall of the tubular member, hydroforming fluid will leak out between the punch and the wall of the tubular member. This will cause a significant drop in pressure, which is undesired.
A second method of piercing used in hydroforming applications is “hydrapiercing”. Upon completion of hydroforming, the tubular member is in intimate contact with the wall of the die. The hydroforming fluid is at a forming pressure, approximately 10,000 psi. A punch is attached to a hydraulically actuated cylinder. When the hydraulic cylinder is retracted, the high-pressure fluid will force the (non-supported) area of the tubular member outward, allowing metal to be sheared and produce a slug. The “hydrapiercing” method is mostly used for piercing large openings. In order to support the forming pressure of approximately 10,000 psi or more, this method requires large hydraulic cylinders, which is undesired. Packaging as well as more hydraulic flow to operate large cylinders is a constant problem in designing hydroforming dies.
As a result, it is desirable to provide a punch assembly to pierce openings in a tubular member during the hydroforming process. It is also desirable to provide a punch that can withstand extremely high forces. It is further desirable to provide a punch assembly that is more compact than a hydraulic cylinder of equal power. Therefore, there is a need in the art to provide a new punch assembly for a hydroforming die that meets these desires.